Receiver selectivity: In most superheterodyne receivers, which block primarily determines the overall selectivity (i.e., adjacent-channel rejection and bandwidth)?
Correct Answer: Characteristics of the IF section (IF filters and IF amplifiers)
Introduction / Context:Selectivity defines how well a receiver separates the desired channel from adjacent channels. In superheterodyne architectures, frequency conversion to a fixed IF allows the use of stable, high-Q filters to set the bandwidth precisely.
Given Data / Assumptions:
- Receiver uses a fixed intermediate frequency (e.g., 455 kHz AM or 10.7 MHz FM).
- IF filters (LC, ceramic, crystal, SAW) provide the bulk of the shaping.
Concept / Approach:
Moving selectivity to a constant frequency means the filter characteristics (bandwidth, skirt selectivity, ripple) are consistent across the entire tuning range. Hence, the IF section predominantly sets receiver selectivity.
Step-by-Step Solution:
Incoming RF → mixer → IF.IF filters shape the passband and reject adjacent channels.Thus adjacent-channel rejection depends chiefly on IF characteristics.Verification / Alternative check:
Service alignment procedures focus on IF filter alignment to correct bandwidth and skirt selectivity, confirming their central role.
Why Other Options Are Wrong:
- Sensitivity relates to minimum discernible signal, not bandwidth.
- Antenna pattern can help but is not the primary selectivity mechanism in superhets.
- Audio and LO power do not define RF/IF bandwidth.
Common Pitfalls:
- Equating sensitivity (gain/noise) with selectivity (filtering).
Final Answer:
Characteristics of the IF section (IF filters and IF amplifiers)